High-pressure EXAFS measurements of solid and liquid Kr

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Abstract
X-ray-absorption measurements of liquid and solid krypton at room temperature in the pressure range 0.1–30 GPa have been performed using the dispersive setup and diamond-anvil cells as a pressure device. The evolution of the near-edge structures as a function of pressure, including the first intense resonance, has been interpreted using multiple-scattering calculations. It is shown that the near-edge structures are reproduced taking into account two-body and three-body terms associated with the first-neighbor atoms. Extended x-ray-absorption fine-structure (EXAFS) spectra have been analyzed in the framework a multiple-scattering data-analysis approach taking proper account of the atomic background including the [1s4p], [1s3d], and [1s3p] double-electron excitation channels. Isobaric Monte Carlo (MC) computer simulations based on empirical pair potentials, as proposed by Barker (K2) and Aziz (HFD-B), have been performed to make a quantitative comparison of theoretical and experimental local structural details of condensed krypton at high pressures. From the analysis of EXAFS data we were able to obtain simultaneous information on average distance, width, and asymmetry of the first-neighbor distribution, as a function of pressure. These parameters yield a unique insight on the potential function because they are affected by both minimum position and curvature of the effective pair potential. The trend of the first-neighbor distribution as a function of pressure is in quantitative agreement with the HFD-B potential at moderate pressures, deviations are found at higher pressures where EXAFS spectra are very sensitive to the hard-core repulsive part of the potential. The weak EXAFS signal of liquid krypton at room temperature and 0.75 GPa has been found in accord with the results of the MC simulations within the noise of the measurement. © 1996 The American Physical Society.